1. Field of the Invention
This invention relates to a fixing device included in an image forming apparatus which adopts an electrophotographic method, an electrostatic recording method, or the like. More particularly, the invention relates to a heating fixing device which includes by a fixing member and a pressing member disposed in pressure contact with each other.
2. Description of the Related Art
Among conventional fixing devices included in image forming apparatuses which adopt an electrophotographic method, an elestrostatic recording method, or the like, so-called heating fixing devices are widely used. In such devices, an unfixed toner image carried on a transfer material is fixed as a permanent image by passing the transfer material through a nip portion formed by a fixing roller and a pressing roller which rotate in pressure contact with each other. In such a heating fixing device, in order to prevent creases in the transfer material produced by passing it through the nip portion, at least one of the fixing roller and the pressing roller is generally configured in a so-called inverse-crown shape, in which the outer diameter at two end portions in the longitudinal direction is greater than has a the outer diameter at a central portion has a. However, the roller having such a configuration forcedly stretches the transfer material in the longitudinal direction of the roller, so that two end portions of the transfer material, which is not rigid, are corrugated, thereby degrading, in some cases, the quality of the obtained image. In order to prevent such corrugation, a proposal has been disclosed in U.S. Pat. No. 5,130,754 (Japanese Patent Laid-open Application (Kokai) No. 2-262684 (1990)), in which the largest-diameter portions of a pressing roller configured in the inverse-crown shape are positioned inside the two ends of the roller to form slight stripes at portions of the transfer material facing the largest-diameter, so that two end portions of the transfer material are strengthened.
However, detailed research conducted by the inventors of the present application have shown that even in the above-described proposal, when the size of the fixing device is reduced so as to provide a personal-use image forming apparatus, the following problems arise.
That is, when the diameter of the fixing device is reduced in order to provide a small image forming apparatus, the mechanical strength of the fixing roller is reduced. Hence, the fixing roller in pressure contact with the pressing roller is bent in the shape of a bow, whereby a greater amount of pressure and heat are supplied to two end portions of the transfer material. As a result, fibers of the transfer material are greatly stretched, whereby a curve of the output transfer material toward its non-printed surface (so-called inverse curl) is apt to be produced. This inverse curl cannot be completely removed even by inverse-curl correction means particularly in an image forming apparatus, in which sheets are frequently discharged in a face-up state, such as a printer for a personal computer. Hence, the mountability of sheets in the apparatus is inferior. When sheets of the transfer material are discharged in a face-up state, the discharged sheets curl, whereby the quality of obtained images are remarkably degraded.
When the diameter of the pressing roller is reduced as well as the fixing roller, the hardness of the pressing roller must have a small value in order to maintain a nip width sufficient for fixing an unfixed toner image on the transfer material. In such a case, even if the shape of end portions of the pressing roller is changed as disclosed in the above-described proposal, the effect of the change is weakened, and "corrugation" is apt to be produced.
Furthermore, it is very difficult, from the viewpoint of accuracy, to finely adjust and form the shape of two end portions of the pressing roller made of an elastic material in a manner as disclosed in the above-described proposal, and therefore the productivity of the device is inferior.
Thus, it has become clear that in the device disclosed in the above-described proposal, the proposed effect is insufficient prevent corrugation at end portions of the recording material. If output sheets having corrugation at end portions are, for example, superposed and bound, ends of the bound sheets are not exactly aligned, thereby presenting an awkard appearance. Such a phenomenon is more pronounced as the outer diameter of the fixing roller is smaller. Recently, this phenomenon is apt to arise more frequently because the diameters of the fixing roller and the pressing roller are becoming smaller as the size of the heating fixing device is reduced as a result of development of personal-use electrophotographic printers, copiers and the like.